PORT ANGELES -- Earthquakes silent to the naked ear are revisiting the North Olympic Peninsula.

Crews are doubling the number of seismic recording devices between Port Angeles and Port Townsend to measure tremors that make their presence known every 15 months or so in northwestern Washington, Pacific Northwest Seismic Center Director John Vidale said Wednesday.

Staff from the University of Washington-based center this week are increasing seismic stations from 10 to 20 at each of eight sites on the North Olympic Peninsula.

The "tremor-and-slip" events have occurred about every 15 months since they were first detected in 2002.

The latest were felt Sunday north of Olympia and west of Tacoma

They are expected to travel north under the Peninsula to Vancouver Island.

The tremors are expected to last about three weeks and chances are "very small" that they will be felt on the surface, Vidale said.

Over the course of several weeks, these silent tremors can release as much energy as a magnitude 6 earthquake, UW scientists said.

They emanate from the Cascadia subduction fault zone about 50 miles off the Washington coast.